To the Editor: Drs. Buist and Vollmer emphasize the role of residential environmental factors in the increasing rates of morbidity and mortality from asthma observed over the past decade. We wish to underscore that exposures in the work environment are also important in precipitating and exacerbating asthma. Exposures at work are estimated to cause between 2 and 15 percent of all cases of asthma in adults. In the United Kingdom, asthma is the most common work-related respiratory condition reported to a national surveillance system, making up 28 percent of cases. Among 13 U.S. clinics specializing in occupational and environmental diseases, asthma and related diagnoses represented 38 percent of all non-asbestos-related occupational respiratory disorders. Asthma can develop in workers after sensitizing or nonsensitizing exposures through the respiratory tract. Sensitizing exposures to a wide variety of airborne chemicals and plant or animal proteins occur in many different industries. Isocyanates alone account for approximately 20 percent of cases of occupational asthma reported in both the United Kingdom and the United States. Irritant exposures occur in numerous work environments. In addition to the precipitation of new cases of asthma, episodes of bronchospasm triggered by exposure at work to dusts, fumes, and other irritants, as well as by extreme temperatures, can occur in patients with preexisting asthma.